1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus for photographing with, e.g. a higher photo-sensitivity.
2. Description of the Background Art
When photographing at a higher photo-sensitivity level, photographing over a longer exposure time, or photographing at a higher temperature, a solid-state image pickup device may produce from its effective photosensitive area an image signal lowered in level. This level lowering may be prevented by switching digital clamping to clamping with signal processing, as is disclosed for example in Japanese patent laid-open publication No. 2003-319267.
In recent years, micropixelation of solid-state image pickup devices, such as a CCD (Charge-Coupled Device) image sensor, has reduced the number of generated electric charges to lower a signal level used. Notwithstanding, cameras replete with a high-sensitivity mode capable of photographing wit less camera shake are called for in the market.
However, in view of achieving a camera replete with a high-sensitivity mode, a significant increase in gain is required in the subsequent stage to the output port of an image pickup device, and thus, a small distortion of the black level causes a visible image to significantly deteriorate, which has been a problem. In the idea disclosed in the above-indicated publication, clamping focuses attention on its noise, but not on noise in the level lowered in the effective area, and it is therefore considered that the advantage is in practice restrictive.
A real problem in clamping signals having much noise, such as those in a high-sensitivity mode, i.e. high ISO (International Organization for Standardization) mode, involves the following case: In a super high ISO mode, such as ISO3200 or ISO6400, a noise quantity to be estimated cannot be disregarded for signals. In such a case, if the processing sequence of the clamp level process, black level correction, i.e. offset correction, and noise reduction should not be adequately controlled, then the signals after reduced in noise cause linearity defects, which leads to, for example, a turned hue or distortion in hue. The present invention relates to an image pickup apparatus for preventing linearity defect in a noise clipping process for processing an output of an image pickup device at the time of, e.g. high-sensitive photographing.